Ion 534a

οἱ μελοποιοὶ οὐκ ἔμφρονες ὄντες τὰ καλὰ μέλη ταῦτα ποιοῦσιν, ἀλλ᾽ ἐπειδὰν ἐμβῶσιν εἰς τὴν ἁρμονίαν καὶ εἰς τὸν ῥυθμόν, βακχεύουσι καὶ κατεχόμενοι,
I am not sure I understand this kai before κατεχόμενοι, would it not be better to leave it out?
also, CGLC commentary says that there is an anacoluthon here because κατεχόμενοι has no subject expressed, for it shifts further to καὶ τῶν μελοποιῶν ἡ ψυχὴ. I don’t understand this, it is much easier to consider οἱ μελοποιοὶ as the subject of κατεχόμενοι .
PS: I have checked Stob., the kai is absent there.

When you suggest leaving it out, it’s not clear whether you mean ignoring it in translation or assuming it’s an interpolation in the text. Certainly it would be grammatically easier if the και were not there, but it seems to me (as evidently to the CGLC scholars) that we’re to recognize an anacoluthon. ωσπερ begins a parenthesis, which is extended by εμφρονες δε ουσαι ού, by which time the interruption is such that the first participle is left behind, or resumed in different form.
Of course, new evidence could put things in a different light.

πάντες γὰρ οἵ τε τῶν ἐπῶν ποιηταὶ οἱ ἀγαθοὶ οὐκ ἐκ τέχνης ἀλλ᾽ ἔνθεοι ὄντες καὶ κατεχόμενοι πάντα ταῦτα τὰ καλὰ λέγουσι ποιήματα, καὶ οἱ μελοποιοὶ οἱ ἀγαθοὶ ὡσαύτως, ὥσπερ οἱ κορυβαντιῶντες οὐκ ἔμφρονες ὄντες ὀρχοῦνται, οὕτω καὶ οἱ μελοποιοὶ οὐκ ἔμφρονες ὄντες τὰ καλὰ μέλη ταῦτα ποιοῦσιν, ἀλλ᾽ ἐπειδὰν ἐμβῶσιν εἰς τὴν ἁρμονίαν καὶ εἰς τὸν ῥυθμόν, βακχεύουσι καὶ κατεχόμενοι, ὥσπερ αἱ βάκχαι ἀρύονται ἐκ τῶν ποταμῶν μέλι καὶ γάλα κατεχόμεναι, ἔμφρονες δὲ οὖσαι οὔ, καὶ τῶν μελοποιῶν ἡ ψυχὴ τοῦτο ἐργάζεται, ὅπερ αὐτοὶ λέγουσι.

I think that ἀλλά contrasts οὐκ ἔμφρονες ὄντες, not the ποιοῦσιν. No anacoluthon, and it also explains the καί.

…and songwriters are like this too, they make these real monster ballads not when they’re thinking, but when they get into the tune and the beat; they really break into it and are carried away…

As you can see I’m reading the βακχεύουσι as restating the non-negated ποιοῦσιν.

Joel you miss the point of difficulty, the participle left hanging in midair after και. It’s not και κατέχονται, which your “and are carried away” implies.
What’s that line about rushing in where angels fear to tread?

What could contrast ἔμφρονες ὄντες other than a participle? οὐκ ἔμφρονες ὄντες ταῦτα ποιοῦσιν, ἀλλ᾽ κατεχόμενοι.

No that won’t work, as I suspect you can see.

You’ll have to be more clear.